Hydrogels represent a desirable class of materials for contact and intraocular lenses. The advantages of such materials can include relatively high oxygen permeability, biocompatibility and comfort. Hydrogels are a crosslinked polymer materials having a certain percentage of water. One class of hydrogel materials used for lenses can be prepared from monomeric mixtures containing hydrophilic monomers such as 2-hydroxyethyl methacrylate or N-vinyl pyrrolidone. The oxygen permeability of these hydrogel materials relates directly to the equilibrium water content of the materials.
A second class of hydrogel materials used for lenses can be prepared from monomeric mixtures containing one or more silicone-containing monomers and one or more hydrophilic monomers. In some cases, the silicone-containing monomer or the hydrophilic monomer can function as a crosslinking agent (a crosslinking agent being defined as a monomer having multiple polymerizable functionalities). Alternatively, a separate crosslinking agent can be added to monomer mixture. Silicone hydrogels typically have a water content between 10 to 80 weight percent. These silicon-hydrogel materials typically will have a higher oxygen permeability than a similar HEMA-based hydrogel.
U.S. Pat. No. 5,034,461 discloses various polysiloxane prepolymers with urethane or urea linkages. Generally, these prepolymers are derived from a short chain diol, a hydroxy-terminated polydimethylsiloxane and a diisocyanate such that the structures resemble a segmented polyurethane elastomer. The polysiloxane prepolymers are then endcapped with polymerizable ethylenically unsaturated radical such as HEMA reacted with isocyanate. The polysiloxane prepolymers can be copolymerized with a hydrophilic monomer to form a silicone hydrogel copolymer that is useful as a contact lens material or other biomedical device applications.
The polysiloxane prepolymers described in U.S. patent application Ser. No. 11/292,817, filed Dec. 2, 2005 comprise soft and strong hard segments as in U.S. Pat. No. 5,034,461, however, the prepolymers further include relatively weaker hard or medium hard segments. The addition of the medium hard segments can provide several material advantages. First, the later prepolymers tend to have a lower viscosity at room temperature, which can allow for easier processing during synthesis and in casting of biomedical devices. Second, the later prepolymers can provide a material with a higher silicone content, thereby resulting in a material with higher oxygen permeability yet maintaining good compatibility with hydrophilic monomer and forming clear hydrogels.
Regardless of their water content, many silicone hydrogel materials tend to have relatively hydrophobic, non-wettable surfaces. Those skilled in the art have recognized the need to increase the hydrophilic nature of the surface of these materials, in particular, for materials used for contact lenses. Increasing the hydrophilicity of the contact-lens surface improves the wettability of the contact lenses with tear fluid in the eye. This in turn improves the wear comfort of the contact lenses. In the case of continuous-wear lenses, the surface properties of the material are especially important. The surface of a continuous-wear lens must be designed, not only for comfort, but to avoid adverse reactions such as corneal edema, inflammation, or lymphocyte infiltration.
One known method to increase the hydrophilicity of the silicon hydrogel surface is with a plasma treatment as described in U.S. Pat. No. 6,630,243. This patent describes a method to provide a carbonaceous layer with a plasma treatment followed by attachment of hydrophilic polymer chains to the carbon layer. Alternatively, hydrophilic or otherwise biocompatible polymeric chains can be attached to a surface of an ophthalmic lens by chemical modification of the surface. For example, U.S. Pat. No. 5,652,014 describes amination of a substrate followed by reaction with hydrophilic polymers such as a PEO star molecule or a sulfated polysaccharide.
The use of siloxane prepolymers for the fabrication of optical lenses is well known due to the relatively high oxygen permeability and softness of the resulting three-dimensional lens materials.